Frequency spectrum is a scarce resource and future communication system tries to utilize the available spectra more efficiently. One example of this is Wideband Code-Division Multiple Access (WCDMA), where the original system was using QPSK (Quadrature Phase-Shift Keying) modulation, generating 2 bits/Hz. The latest version of the WCDMA, eHSPA (High-Speed Packet Access), uses 64 QAM (Quadrature Amplitude Modulation) to get 6 bits/Hz. Future communication systems will use even higher order modulation (up to 1024 QAM is discussed already now). To maximize the usable data throughput, very low coding overhead is used. To be able to detect the signal without bit errors, the imperfections in the radio needs to be very small. The imperfections include thermal noise, IQ phase and gain mismatch, dc offsets, Local Oscillator phase noise and digital quantization noise. This invention discusses the Local Oscillator phase noise part, which could be a dominating part of the imperfections.
Most modern transceiver solutions use direct conversion receiver and transmitter architectures to reduce complexity and minimize power consumption. Such transceivers use local oscillators responsible for frequency generation in the receiver as well as in the transmitter. The phase noise of the local oscillator signal will be superimposed on the receive/transmit signal and degrade the signal quality. Since the receiver will filter the received signal before entering the detector, only the phase noise within +−fch/2 is of interest, where fch is the bandwidth of each channel.
The local oscillator output has to be a clean low noise signal to get good quality reception/transmission. This is normally achieved by locking an LC-oscillator to a crystal oscillator using a phase locked loop. The LC-oscillator needs to cover all desired RF RX/TX frequencies. The phase locked loop is typically delta-sigma controlled, and includes a crystal oscillator, a phase frequency detector, a charge pump, a loop filter, a voltage controlled oscillator, a frequency divider and a delta-sigma modulator.
Studies of the noise properties of a delta-sigma controlled phase locked loop show that the charge pump is one of the dominating imperfections in the phase locked loop. The charge pump imperfections can be divided into two noise contributors:                Thermal noise and 1/f noise.        Noise problems due to nonlinear transfer function of input phase to output charge. This nonlinearity will cause folding of the high pass filtered delta-sigma quantization noise into in-band phase locked loop noise.        
Normally, the charge pump is implemented using MOS current sources. To get good repeatability of the phase locked loop bandwidth, the current is generated using a band gap reference. The band gap reference is used to generate a reference voltage insensitive to temperature and supply voltage. This voltage is then converted into a current using a resistor. Finally it is scaled to the desired output current in a ratioed current mirror. By using a programmable current setting resistor and programmable frequency divider, the phase locked loop dynamics can be controlled. Finally, the charge pump is controlled from the phase detector using UP and DOWN signals.
When using a charge pump in a delta-sigma controlled phase locked loop, it is critical to have good matching between the sink and source current sources. Otherwise the phase locked loop noise performance will suffer.
When implementing such a charge pump, several tradeoffs have to be made. First of all, the solution is inherently noisy, since a lot of noise sources are present. Noise can be improved at a current cost.
Also, when implementing the current sources in a deep CMOS process, nmos-pmos-devices are used to implement the current sources. To get good matching between the sink and source current pulses, the mos current sources have to be sized large.
Also, since the output voltage will be varying, the output impedance of the current sources needs to be high. This calls for long devices/cascoding. It is difficult to cover a wide loop voltage range with good sink/source matching.
The above problems call for large sized devices, which will cost current to drive. A worse problem is that it causes slow rise/fall times and in particular mismatch between sink and source rise and fall times, since different devices are used (nmos vs. pmos). This also causes delta-sigma noise folding, which degrades phase locked loop noise performance further.
There is a need for a fast, well-matched and low noise charge pump.
An article, Brownlee et al “A 0.5 to 2.5 GHz PLL with Fully Differential Supply-Regulated Tuning”, Proc. ISSCC 2006, pp. 588-589, addresses the charge pump noise problem by using a ring oscillator VCO with differential tuning input, which is not the case with an LC-based VCO. This solution suggests a way of generating a “charge-pump” current via resistors by connecting them to a mid-reference common-mode, virtual ground node. It requires two loop filters, of which one is an active loop filter, which incurs an operational amplifier in the signal path and further degrades noise. The use of an operational amplifier in the loop filter probably degrades noise more than what is gained by using resistors. Further, the use of two loop filters doubles loop filter noise and area.